1. Field of the Invention
This invention relates to water soluble polymers that have both a very high molecular weight and contain both water soluble monomers and a water insoluble monomer, and a method of producing them. More particularly, it relates to a process for the polymerization of water soluble acrylamide monomer and a water insoluble higher alkylacrylamide monomer, such as octylacrylamide. The process relies on solubilizing the water insoluble monomer into a micellar aqueous solution of the water soluble monomer and of a surfactant in order to effect the copolymerization. A redox catalyst mixture at low concentrations is used to initiate the polymerization at a low temperature. The polymer may also be post-treated by the addition of base in order to produce a partial hydrolysis of the acrylamide to give a metal salt of acrylic acid, thereby converting it into an anionic polymer that has the characteristics of a polyelectrolyte. Alternatively, the anionic groups may be incorporated into the copolymer during the polymerization by, for example, including in the reaction mixture acrylic acid, acrylamide and the higher alkylacrylamide.
2. Description of the Prior Art
Polyacrylamide and hydrolyzed polyacrylamide are well-known water soluble polymers useful for water treatment, paper strengthening agents, dewatering sludge and for controlling the viscosity of water or brine for secondary or tertiary oil recovery. Some of these processes require polymers having a very high molecular weight for optimum effectiveness. A particularly important use that relies on the molecular weight of the polymer for effectiveness is the viscosification of water or brines.
The preparation, characterization and solution properties of polyacrylamide are surveyed by W. M. Kulick, R. Knwieski and J. Klein, Prog. Polym. Sci., I, 373 (1982) by W. H. Montgomery in Water-Soluble Gums and Resins, 2nd Ed., R. L. Davidson and M. Sitig, Editors, Van Nostrand Reinhold, New York, 1968, chapter 9; and by P. Molyneux, in Water Soluble Synthetic Polymers: Properties and Behavior, Volume II, CRC Press, Inc., Boca Raton, Fla., 1983,chapter 3.
Technically the term "polyacrylamide" refers also to products which are obtained by the hydrolysis of polyacrylamide or by the copolymerization of acrylamide with acrylic acid. Processes for preparing polyacrylamides and related polymers are well-known in the art. Minsk, et al., U.S. Pat. No. 2,486,191, for example, teaches the polymerization of acrylamide and methacrylamide in mixtures of water and a water miscible alcohol to control the molecular weight of the product. Suen and Schiller, U.S. Pat. No. 2,820,777, teaches the continuous polymerization and hydrolysis of acrylamide to polymers containing both amide and carboxylate groupings whereby the degree of hydrolysis and degree of polymerization may be readily controlled. Friedlander, in U.S. Pat. No. 2,886,558, describes the use of an alkali-metal hydroxide to effect a controlled conversion of the amide groups of a preformed polyacrylamide to yield a partially hydrolyzed polyacrylamide. The preparation and characterization of partially hydrolyzed polyacrylamides are also given in a paper by J. Klein and R. Heitzmann in Makromol Chem., 179, 1895 (1978).
While the object of many of these patents is to produce high molecular weight polyacrylamides for optimum efficiency, it is still very desirable to further extend the range of molecular weight available for the polymers and, thereby, enable a decrease of the concentrations required for various uses. This approach to improved properties may be, however, limited by other factors. For example, it is well known that polymers with higher molecular weight degrade more readily under shear, such as might be encountered in pumps and mixers, and also generally dissolve more slowly than lower molecular weight polymers.
An alternative method for making polyacrylamides that are even more effective has been described in U.S. Pat. Nos. 4,521,580 and 4,528,348. These patents describe the incorporation of a small mole fraction of water insoluble monomer into the water soluble polymer during the polymerization. The hydrophobic groups tend to associate with one another in an aqueous solution, resulting in an increase in viscosification efficiency relative to a polymer without the hydrophobic side groups. The polymers produced by these methods, which are disclosed in U.S. Pat. No. 4,520,182, are nonionic and, therefore, have the advantage that they are relatively insensitive to the presence of salts in the water. However, this previous art has not been able to produce polymers having sufficiently high molecular weight that the viscosification efficiency at infinite dilution (i.e., the intrinsic viscosity) is comparable to or higher than that achieved by the commercially available biopolymer (xanthan) or hydrolyzed polyacrylamide. The latter have intrinsic viscosities at a shear rate of 1.3 s.sup.-1 of about 80 and 25 dl/g, respectively, while these patents describe only copolymers having intrinsic viscosities from about 1 to about 7 dl/g.
The possibility of producing copolymers of acrylamide and higher alkylacrylamides is also suggested in such prior art as Lenke, et al., U.S. Pat. No. 4,151,333; Barua, et al., U.S. Pat. No. 4,098,987; Evani, European Pat. Application 0057875; and Emmons, et al., U.S. Pat. No. 4,395,524. Landoll, U.S. Pat. No. 4,304,902, describes copolymers of ethylene oxide with long chain epoxides which require relatively large polymer concentrations (above about 1%) for thickening water and require surfactants for solubility due to irregularities in the polymerization. In a related case, U.S. Pat. No. 4,428,277, modified nonionic cellulosic ether polymers are described. The polymers claimed in each of these applications are either water insoluble or have such a low molecular weight that they have limited usefulness for the viscosification of water or brine. The processes used to produce them are also quite different from that described in the present invention.
One process suggested by this prior art (Lenke, Emmons and Evani) is to cosolubilize the monomers in a solvent or solvent mixture having solubility properties intermediate between water and a hydrocarbon solvent. Although this undoubtedly allows the comonomers to come into close proximity to one another, since the dispersion is on a molecular scale, this process presents difficulties. For example, often the copolymer is insoluble in the mixed solvent which is capable of solubilizing the monomers. This leads to precipitation of the copolymer when it has a molecular weight too low to produce efficient viscosification. In addition, the reaction product is usually heterogeneous and, therefore, requires additional processing steps. Moreover, the water miscible solvents, such as alcohols, acetone, ether and acetic acid, which are used are good chain transfer agents and when used in reasonable quantities leads to the decreased molecular weights and, hence, to the poor viscosification efficiency that these materials exhibit.
Another method used to copolymerize water soluble and water insoluble monomers uses a conventional emulsion polymerization process to suspend the water insoluble comonomer in an emulsified form by the use of a dilute aqueous solution of a water soluble surfactant. Here the droplets of the water insoluble monomer are at least one micron in diameter. The product, as exemplified by Barua, U.S. Pat. No. 4,098,987, generally is a copolymer in the form of particulates with diameters on the order to 500 to 2,000 .ANG. in diameter. The water insoluble copolymer so produced has a negligible efficiency for viscosifying water or brine.